I am experiencing an under the seat vibration almost like a vibrator at approx 35mph and then it quickly goes away as I increase the speed. Does this sound like U-joints or something more serious?
Besides checking the u-joints as suggested, I'd look closely at the rear tires. When front tires have a issue you feel it in the steering wheel, rear tires you feel in you seat. Tread separation would do it, but that typically separation creates a sensation at a lower speed. Tires that have developed bad "heel-toe" wear could cause your symptoms. Often times you can see it, but if you run your hand over the tread it'll be smooth in one direction, but "catch" in the other.
If you rotate the tires and the sensation moves to the steering wheel that'd be another way to isolate the issue. If rotating doesn't move or change the vibration than start looking elsewhere.
If you do have heel-toe wear its usually from a few factors. Not rotating the tires enough, or tires just too hard. The back ends of Toyota trucks are fairly light when empty, and if the tire material is on the harder side the light bed, stiff suspension and hard tires will sometimes let the back end hop a bit, just enough to cause the uneven wear. Tire shops often will fault alignment or shocks(instead of the tires they recommended or installed), but it really just comes down to tire composition.